ALBERT

Description: The lower Miocene to Pleistocene at Sites 642 and 644 is subdivided into eight silicoflagellate zones and nine ebridian-actiniscidian zones. Due to the bioevents selected, the silicoflagellate zonation may be recognized in high northern and southern latitudes. A sequence of 24 diagnostic silicoflagellate, ebridian and actiniscidian events tied to absolute ages is established, and both the biozonations and the event sequence are correlated with standard nannoplankton zones and paleomagnetic anomalies. Based on fluctuations of cool and warm water-preferring silicoflagellate taxa, more than ten cooling phases of seasurface waters are recognized for the early Miocene to late Pliocene at Site 642. A relative paleotemperature curve derived from same data indicates that sea-surface temperatures were relatively high from 21.7 to nearly 13.4 Ma. Temperatures decreased progressively during the late Miocene until a distinct warm peak around 5.45 Ma. During the Pliocene sea-surface temperatures were generally low and dropped drastically at about 3.2 Ma, just before the final disappearance of silicoflagellates at Site 642 around 3.1 Ma, probably corresponding to a brief glacial phase. At Site 644, silicoflagellates, ebridians, and actiniscidians persisted from 2.8 until 2.57 Ma, when abundances suddenly decreased with the onset of a substantial Northern Hemisphere glaciation. After an interval of disappearance from about 2.5 to 2.1 Ma, siliceous flagellates reappeared at Site 644 during several phases of warm-temperate water incursions from the North Atlantic. At about 1.9 Ma ebridians disappeared at Site 644, and at 0.74 Ma silicoflagellates and actiniscidians also disappeared. Abundance drops and disappearances of silicoflagellate and ebridian species from the early Miocene to late Pliocene proceeded during cooling phases. The most prominent events were the abundance drops in Corbisema and Foliactiniscus species and the disappearance of all Naviculopsis species in the early Miocene, and the successive disappearances of many silicoflagellate and ebridian species during the late Miocene and Pliocene. Seven new silicoflagellate taxa (Cannopilus hemisphaericus f. heptagonus, Dictyocha fibula subsp. tenuis, Distephanus paraspeculum f. paraspeculum and f. hexagonalis, D. quinarius, D. speculum subsp. constrictus, D. sulcatus f. maximus), six new ebridian species (Falsebria arborea, Haplohermesinum hovassei, Hermesinella primitiva, Pseudammodochium fenestratum, Spongebria curta, S. miocenica), and two new actiniscidian species {Actiniscus planatus, Foliactiniscus pulvinus) are described from Sites 642 and 644.